1. Field of the Invention
This invention relates to metal-air batteries and more particularly to a movable anode design for a metal-air fuel cell battery for improved charging and discharging of the anode.
2. Description of the Related Art
Some of the problems in the past with recharging metal air systems were due to shape change of the anode, densification of the anode and dendrite formation on the anode. These anode related problems limited the life of the rechargeable system. Solutions to these problems typically involved decreasing current density (for both discharging and recharging) and depth of discharge. Both of these side-effects severely cripple the metal-air system's chance of having good energy and power densities.
Metal-air batteries have been limited in the past because there has been a trade off between high energy/power densities and good charging characteristics.
Another limiting factor in the past has been finding a bifunctional air electrode which is efficient for both recharging and discharging.
Shape change is related to the lifetime of the system. When the shape changes during each recycling the capacity of the system decreases significantly and also will cause some shorting problems.
One attempt to solve the shape change problem used a reticulated sponge-like zinc anode which increased the surface area of the zinc (decreasing current density). The lowered current density decreased the energy density of the system. Further, the reticulated sponge-like zinc anode did not prevent dendrite growth.
Dendrites grow from the anode, reach through the separator, and touch the air electrode which shorts out the cell.
Attempts to limit dendrite growth on the reticulated zinc anode included using a chemically inert coating on the exterior of the anode. This reduced the dendrite growth but the loss of anode area lowered the capacity of the cell.
Anode shape change was combated using a pump to circulate the electrolyte. By continually stirring the electrolyte a more uniform distribution of zinc ions in solution will result. A uniform mixture of zinc ions in the electrolyte will greatly reduce shape change to the anode over repeated cycling.
Another attempt to limit shape change and dendrite growth was by L. R. McCoy and L. A. Heredy in 1972 (U.S. Pat. No. 3,663,298) whereby zinc pellets and electrolyte were used to fill about 2/3 of the volume of a circular rotating drum. One of the walls of this drum was the air electrode. The drum would rotate during discharging and recharging, and the zinc particle bed would continually mix within the cell. Because the particles could move freely, fresh zinc would continually and evenly be exposed to the air electrode. This provided a longer discharge life at higher current densities by providing even depositing of zinc during recharging.
The rotatable electrode had improved rechargeability characteristics. It was found possible to recharge and discharge the rotating electrode repeatedly at rates up to 100 mA/cm.sup.2. Conventional zinc electrodes do not ordinarily withstand recharge rates in excess of 20 mA/cm.sup.2 on repeated cycling without rapid failure by dendritic shorting. The high recharging rates were possible because the continual movement of the particle bed provided for a smooth, dendrite free, zinc coating on the pellets.
The rotatable electrode improved on conventional zinc/air technology, but required the use of an inefficient bifunctional air electrode.
Bifunctional air electrodes have very low cycle numbers because the electrode has to be used both for charging and discharging. It is very difficult to optimize such an electrode to function efficiently for both actions. In the past people have tried using many different catalysts and different electrode structures for bifunctional air electrodes, but the lives of the rechargeable zinc air systems are still severely limited.
Another attempt at solving the problems associated with recharging metal/air systems was in 1971 (see Fuel Cell and their Applications published in 1996, pg. 160). Sony corporation constructed a zinc/air cell containing a third electrode. The cell comprised a zinc anode sandwiched between one recharging air electrode and one discharging air electrode. The idea was to eliminate the need for a bifunctional air electrode. The zinc anode would be discharged from one side and recharged from the opposite side which optimized each electrode independently.
Sony's zinc/air cell was an improvement on the bifunctional air electrode. However, the zinc anode could only be discharged from one side, which cuts in half the power capabilities of the cell. Further, the zinc anode is charged from the side where it was discharged the least; which decreases the efficiency of the system.
Another problem with the Sony design is that the anode has to be a porous structure so that the electrolyte can flow from the discharge side to the recharge side to provide ions in solution from discharging in order to recharge again.
A patent to Faris U.S. Pat. No. 5,250,370 in FIGS. 8 and 9 shows a rotating anode with one air electrode on one side of the anode. This is another bifunctional air electrode and it only discharges and recharges on one side of the anode disk.